Wednesday, 22 August 2007

Loud Shirt

I recently appeared on my local TV station. When I arrived at the studio I was asked to change my shirt because its pattern would look distorted on screen. What causes this effect and why, in this day and age, is it impossible for television to simply record what is in front of the cameras?Alan Francis, Cardiff, UK

There are two possibilities. First, analogue television in the UK uses PAL colour coding, which transmits the colour information as very fine luminance patterning on a simple black-and-white signal. If you feed a colour TV signal into a high-resolution, black-and-white monitor, strong blues and yellows appear as fine backward-slanting stripes and strong reds and greens as forward-slanting stripes. Mixed colours, such as orange or cyan, make cross-hatch patterns. A colour TV receiver can mistakenly decode fine-patterned shirts as encoded colours which flicker and are generally annoying. The decoders in newer TVs are much better than those of the 1960s when the system was invented, but it is a fact of the coding standards that colour and luminance cannot be fully separated under all circumstances.

The second possibility relates to the bandwidth of the broadcast signal. Studio-grade digital television operates at a data rate of 270 megabits per second, but for digital broadcast by satellite this signal is compressed to between 2 and 5 megabits per second. To achieve this, many of the 25 frames that are shown per second are not transmitted in their entirety; instead, the differences between one frame and the next are sent. Busy patterns on clothing can cause stress in the part of the coder that monitors changes from one frame to the next, resulting in shimmering of the cloth and degradation of the entire picture because too much data capacity has been used on the cloth.

Broadcasters now transmit four or five digital TV stations in the channel previously occupied by a single analogue programme, so it is hardly a surprise that something gets lost along the way!

A couple of images on my website show luminance/chrominance coding patterns: click here. You can also find the BBC's research and development group's white paper on digital TV compression issues here.

If it has a complex pattern, it may shimmer annoyingly because of the phenomenon of "aliasing". Imagine stripes that, when appearing on the screen, are closer together than half the spacing of the screen pixels. The TV cannot resolve the stripes because it doesn't have the resolution. This is also why wagon wheels appear to go backwards in movies. The problem can be reduced by higher-resolution TV but will never be completely eliminated.

Certain very bright colours, known as "hot colours", can fall outside the range of colours that can be encoded by the TV signal. If your shirt is extremely loud, this could be the problem.

Finally, if the TV studio was using a blue or green screen and your shirt contained a colour similar to the screen, this would have the effect of making your body transparent, which would be embarrassing.

An earlier answer to this question (19 May) suggested that wagon wheels appear to go backwards in movies and on TV due to the finite horizontal resolution of the camera/TV system.

In fact, this illusion is caused by the strobing effect of the frame rate. Essentially, each frame (25 or 30 per second depending on the country) takes a snapshot of the position of the spokes and if the rate of movement of the wheel is such that the next spoke in turn hasn’t reached the position of the previous spoke by the time the next frame is taken the brain will perceive a reverse rotation.

The first is called the moiré effect. A television image is made up of horizontal scan lines and certain patterns, especially tight vertical lines on shirts will create an inteference pattern. Lots of detailed explanations on the internet under Moiré.

The second problem is the colour red which older televisions had great trouble reproducing. Any red object would bleed into other colours creating blurry edges around it.

The third dilemma is video has a hard time with high contrast images. White objects in a shot (like shirts) reflect too much light which requires the brightness to be brought down which consequently darkens the subject's face. When the subjects face is properly lit white shirts can "blow out" the image creating an IRE level (luminance) that's too high. This is particularly true in television studios where lighting is pre-set, as opposed to on location shoots where lighting techniques can be used to compensate.

Technology has gone a long way towards lessening these problems, especially high definition, but most productions still follow the rules for shirts of no tight patterns, no reds, and no white or black shirts.

If the pattern on your shirt was narrow stripes then the reason the shirt would be distorted on-screen could be due to pixels. The stripes, if narrow enough can sometime appear distorted if in line with the pixels (ie. sitting parallel with the camera). You can try this for yourself by hooking a video camera up to your TV. Point the camera anything with narrow stripes (a shirt!) and look at the television screen. Now rotate the camera round so that the stripes are going diagonally across the screen. Rotate back and forth and you should see the stripes distort.

I don't quite understand how moiré patterns can appear in real film(as opposed to rasterized or raytraced CG images).

Each pixel element on a digital camera´s CCD has an area, it is not a 'point sample', it averages the light input over some small solid angle. Intuitively this would have the effect of averaging away high-frequency components above the nyqvist limit(just as is done with mip-mapping in 3d graphics.).

Is this an effect that only appears in 'colour' CCDs where there are 3 different types of elements sensitive to different frequencies of light?

Wouldn't it help a lot to use a higher resolution camera and downsample?(so that each colour element is more widely dispersed over the pixel element and less likely to miss high frequency components)

Wouldn't it help a lot to manufacture each square pixel element as 3-square spirals of red, green and blue; so that each colour component will be able to better "see" and average over high frequence components?(if we can print 45 nm transistors it seems likely we can print CCDs in all kinds of interesting patterns with ease).

Smaller pixels will not always help. The Moire' pattern shifts high spatial frequencies to low spatial frequencies, and very fine patterns sampled with very fine pixels will be blown up into the same large, disturbing patterns. The usual technique is to filter the optical signal, and this can help with the problem.

As for making 45nm camera pixel shapes, there are a number of major problems. For starters, red light has a wavelength around 700nm, so it isn't going to pass into your small structures or it will leak directly into adjacent structures. Camera pixels also require transistors, wires and other structures around and between the pixels. With small pixels or pixels with complicated shapes, these structures take up more space. If the light sensitive area is small relative to these other structures, the aliasing problem can get worse. In addition to taking up more space, they will have electronic interactions with each other. Pixels coiled into spirals will leak electrons into one another.

An additional problem for making very tiny or unusual pixel shapes is that not all of the pixel is made of silicon. The pixels need colored filters and micro lenses made of other materials, and these are more difficult to make in the shapes that you would like. Also, the pixel is a three dimensional structure. If you made a thin little spiral, light that entered the pixel at a slight angle would pass through the wrong colored filter before it landed in the "well" below. I can give you an even longer list of problems, but this should give you a feel for the engineering challenges.

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